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Higher level theory involves Bernoulli in lift explanation but attributes difference in velocity between above-wing and below-wing flows to circulation rather than equal transit time All air movements are circulatory Circulation begins to develop when an airfoil is started into forward motion As indicated in the next figure air following the surfaces is accelerated downward Because of downward acceleration pressure below the airfoil is increased and pressure above it is decreased In beginning response to the pressure difference upward recirculation of air mass occurs around the leading and trailing edges As forward airfoil movement continues downward movement left behind the trailing edge divides into forward and rearward rotational patterns carrying upward recirculation around the aifoil and around the center of a receding aft vortex created in shear between downward and upward movements Forward recirculation around the airfoil is known as circulation while the recirculation pattern left behind is known as the starting vortex The principle of conservation of angular momentum demands angular momenta of the two rotational patterns be equal and opposite In equivalent classical teaching Helmholtz’ vortex theorems require equal and opposite vorticity Circulation is regenerative Pressure difference produces circulation and as circulation upward momentum ahead is intercepted by wing forward movement and recurved downward more pressure difference is produced Thus circulation increases regeneratively until reaching the limit at which it provides downward movement at the rear for flow to depart the trailing edge in the pointed direction Circulation in excess of this would be opposed by airfoil direction With mature circulation providing downward movement matching the need for flow departure in the pointed direction at the trailing edge participation of the starting vortex is no longer needed or accepted and growths of starting vortex and forward circulation cease In this mature and stable circulation state called the Kutta condition theoretical lossless lift of an airfoil is equal to the rate of circulation downward momentum produced plus an equal rate of upward recirculation momentum intercepted by airfoil forward movement For derivation of the equation for ideal lift without losses in two-dimensional flow click here Evidence of the regenerative nature of circulation and lift can be sometimes be seen in flow downstream from bridge pilings A round piling has no departing flow direction limiting feature This allows circulation and lift to increase until stall occurs which is followed by lift reversal due to effects of residual boundary layer and pressures A trail of alternating vortices occurs as circulation and lift periodically produce stall and reversal In each reversal of circulation and lift a new starting vortex is carried downstream For more on regeneration of circulation of round cylinders click here Circulation is sometimes described as a rotational movement added to or superimposed on passing flow With the added rotational component oncoming flow can be described as rising upward ahead of an airfoil or wing descending behind having increased velocity above and decreased velociy below Alternatively circulation can be considered as a transitory rotation of air which travels with the airfoil or wing through otherwise relatively still air in a manner somewhat analogous to movement of water as it is parted laterally by a ship bow and rejoins at the stern in semicircular-like movements Such water movements travel with the ship even though the water does not The concept of superimposed circulation may seem a bit abstract but circulation is a real rotational movement that travels through the air with the wings of an airplane in flight Alternate perspectives of fixed wing in passing flow and moving wing through still air are illustrated below In passing flow perspective flow curves upward ahead of the lifting wing or airfoil toward reduced pressure above and away from increased pressure below accelerates rearward into reduced above-wing pressure with increased rearward velocity and decreases rearward velocity below in response to increased pressure At the trailing edge upper and lower flows merge to leave in downward direction pointed by the airfoil Merging into common direction and pressure upper and lower flow velocities difference ceases to exist Downward departing flow curves upward toward being parallel to the path of flight again in response to gradients of decreased above-wing pressure and increased below-wing pressure In stable flight circulation kinetic energy is recovered from flow left behind and imparted to oncoming flow ahead through fore and aft pressure gradient couples and Bernoullian process In lossless stable two-dimensional lift condition equal opposing forward and rearward couples have no net effect on circulation angular momentum which remains constant If angle of attack is increased then downwash demand is increased for re-establishing the Kutta condition Increase is momentarily shared between circulation increase and production of a new starting vortex Angular momentum increase in wing circulation is matched by equal and opposite angular momentum production in the new vortex When the Kutta condition is again established growths of starting vortex and circulation cease and the starting vortex no longer involved with circulation is left downstream In the opposite case of decreased angle of attack a portion of airfoil circulation in excess of that permitted by the Kutta condition is deflected by the airfoil and carried downstream as a new vortex of angular momentum equal in magnitude to that of circulation reduction and of same direction as circulation In two-dimensional flow of wind-tunnel wall-to-wall confinement of an airfoil section circulation and starting vortex develop as two opposite-direction vortices formed in division of downwardly driven air into forward and rearward paths of upward recirculation In this case the stabilized rate of upward recirculation ahead is equal to the rate of downward displacement left behind In the case of a real wing beginning lift in open air where there is no barrier to lateral loss around wing ends circulation starting vortex and wing end vortices are all part of a closed loop of recirculation vorticity as indicated below formed around downwardly accelerated air behind a wing Conservation of angular momentum is maintained as angular momentum on one side of the vortex loop is matched by equal and opposite direction angular momentum on the other side For our purposes air driven downward by a wing or airfoil will be termed downwash and upward recirculation will be termed upwash For a real wing in stable flight upwash is driven upward ahead of the wing and around wing ends by pressure difference between above-wing and below-wing regions Upwash around wing ends in combination with downwash left behind produces twin vortices which trail behind wing ends The starting vortex portion of the initial loop is left behind at the beginning of trailing vortices and has virtually no lasting involvement in wing performanceContinuous production of trailing vortices involves continuous energy loss A helicopter which also derives lift from producing downwash leaves a similar pair of trailing vortices behind but helicopters can fly much slower than fixed-wing craft of comparable weight and thus can leave much more intense vortices The following picture courtesy of NASA shows a vertical column of red smoke drawn into a recirculating downwash vortex left behind a slow flying fixed-wing cropduster If circulation changes as necessary to accommodate changing lift conditions strength of trailing vortices equally changes and a new starting vortex is left behind equal and opposite in angular momentum to change of angular momentum in wing circulation Thus conservation of angular momentum is maintained The body of downward air propulsion and upward recirculation is extensive involving a large rate of air mass in wing circulation and trailing vortices Inertial opposition of the large rate of air mass involvement to downward acceleration allows the required rate of downward air mass acceleration required for lift to be achieved with a sufficiently low effective downward velocity that the rate of kinetic energy input proportional to mass times velocity squared production is small enough for practical flight In level flight the downward momentum produced in lift is ultimately intercepted by the Earth’s surface thereby transferring airplane weight to the surface However a steeply banked turn may produce mostly horizontal momentum which would be retained in continued atmospheric movement That wing lift can be effective in maneuvers of any attitude demonstrates the fact that lift at altitude is in reaction to acceleration in a large mass of circulation not necessarily dependent on weight force transfer to the ground Pressure gradients which produce rising circulation ahead of a wing also produce upward recirculation around wing ends leaving trailing vortices behind In wind tunnel two-dimensional flow airfoil tests trailing vortices are totally prevented by confinement of tunnel walls For a wing operating in open air limited confinement is effected by inertial resistance of surrounding air mass to acceleration Longer wingspan produces greater lateral path length with greater mass inertial resistance to lateral acceleration and associated performance loss The following figure indicates relative lift performances of equal area wings with differing span Differing span is represented in differing aspect ratio the ratio of span to chord INDUCED DRAG The lift process also produces forward and rearward thrust components as the force vectors here indicate Forward thrust results from leading edge pressure reduction in the rearward curvature of rising circulation ahead This pressure reduction known as leading edge suction varies with the ratio of lift to airspeed and on some airplanes is piped from a leading edge port as shown below to a stall warning horn in the cabin Another type of stall sensor actuates when intense circulation around the wing leading edge lifts a small vane also shown which operates a switch connected to an electrical warning horn In reaction to downward and forward acceleration in redirection of circulation at the rear of a wing lift and rearward thrust are generated If there were no losses the rearward thrust would be equal to the forward thrust generated in leading edge suction associated with circulation However the rate of rising air mass ahead of a wing is less than that of descending air mass behind because of partial upward circulation diversion into lateral loss paths around wing ends Thus in addition to loss of lift lateral loss around wing ends causes drag known as induced drag due to rearward thrust of aft circulation redirection being greater than the forward thrust Longer wing spans which provide greater air mass inertial opposition to lateral loss are aerodynamically desirable but limiting factors are stress due to bending moments and space requirements for ground operations and hangaring For special airplanes like Voyager which flew around the world non-stop and unrefueled and the U2 spyplane which flew long high altitude missions long spans were vital Circulation theory is traditionally presented in a highly mathematical context for aerodynamicists and aeronautical engineers as appropriate for aircraft engineering Mathematical complexity and departure from Newtonian logic reduces the utility of most circulation theory books for individuals seeking basic conceptual understanding of wings and aircraft configuration When it was found that mathematical expression of vortex flow velocity involved the same equation as that expressing magnetic field strength induced by electrical current trailing vortices behind wing ends became described as induced The physical basis for induced flow velocity is elusive and authors have written different concepts of it based on differing interpretations of a statement of Ludwig Prandtl 1875-1953 Prandtl stated: The wingtip vortices cause a downward motion of air at the wing which will be shown to be responsible for the drag From Prandtl-Tietjens APPLIED HYDRO-AND AEROMECHANICS ISBN 0-486-60375-X Clark Millikan in Aerodynamics of the Airplane Wiley and Sons 1941 states: The numerical computations in Prandtl’s theory are identical with calculations developed long ago in the theory of electromagnetic induction so that the adjective induced has been taken from the electrical field and introduced into aerodynamics John D Anderson Jr in Fundamentals of Aerodynamics 2nd edition 1991 ISBN 0-07-001679-8 says of trailing vortices: The two vortices tend to drag the surrounding air with them and this secondary movement induces a small velocity component in the downward direction at the wing Krisnamurty Karamcheti in Principles of Ideal Fluid Aerodynamics 1996 ISBN 0-089874-113-0 writes: It is customary to refer to the velocity at any point in the vortex flow as the velocity induced by the vortex It must be understood that this is simply a matter of convenience and does not mean that the vortex is actually causing the flow for they just coexist Clearly there is no common understanding of what induction or induced really means in aerodynamics context Reasonably that is because the concept is at best an analogy TILT OF LIFT DIRECTION In classical aerodynamics which regards trailing vortices as induced from central cores rather than caused by lateral loss of circulation upward movement addition of induced downwash to flow is said to cause downward tilt of flow passing the wing With lift being aerodynamic force perpendicular to relative flow downward tilt of passing flow can account for a rearward tilt of lift illustrated symbolically below The rearward component of tilted lift appears as drag known as induced drag because it is said to be caused by induction effects The concept of drag due to downward tilt of passing flow has merit but the tilt can better be described not as due to addition of induced downwash but due to loss of circulation upwash ahead of the wing as it partially diverts into lateral paths around wing ends making the upwash arrival angle at the wing leading edge less steep than the downwash angle at the trailing edge A MODEST PROPOSAL Obfuscation might have been avoided if the electromagnetism analogy of induction not been so convenient as a substitute for Newtonian reasoning Trailing vortices as well as other aspects of aerodynamic lift should be accounted for in Newtonian terms as proposed by Dr Jaako Hoffren of Helsinki University of Technology in paper AIAA 2001-0872 Quest for an Improved Explanation of Lift presented at the Aerospace Sciences Meeting amp Exhibit in Reno in 2001 If aerodynamics were commonly described in Newtonian terms omitting induction and regarding all air mass accelerations as associated with forces of pressure difference then high school and early college physics and mathematics courses might be enriched with basic aerodynamics topics and false teaching of equal transit time half-venturi theory and electromagnetism-like induction might be abandoned Another questionable point in classical teaching is that of describing circulation as a reaction to the starting vortex which is usually said to be caused by viscous drag at the wing trailing edge The reasoning involves theorems of vorticity which require that creation of any vortex be accompanied by other opposite and equal vorticity This is equivalent to the Newtonian principle of conservation of angular momentum This requirement does not justify assumption that one vortex causes another of opposite rotation as indicated by some authors Starting vortex and circulation can readily be explained as created simultaneously in equal and opposite reactions to pressure gradients occurring when air mass is displaced by airfoil movement Two books by this author present basic aerodynamics in terms of Newtonian principles as taught in high schools and college with circulation but with less mathematical complexity than usually associated with circulation teaching The first soft cover Stop Abusing Bernoulli!- How Airplanes Really Fly ISBN 0-9646806-2-9 with 160 pages and 100 illustrations as shown on the left was published in 1999 The book has sold to many countries around the world and as shown on the right has been translated into Korean by Dr SK Lee for use in Pusan National University Peter Garrison’s review of Stop Abusing Bernoulli! in July 1999 FLYING MAGAZINE recommends it to all pilots and would-be pilots However it is no longer in print although a few copies are still on the shelf at Academy of Model aeronautics AMA museum phone 765-289-4236 A second book with more detail better illustrations and hardcover incorporates the Stop Abusing Bernoulli information and more The second book Introduction to Aerodynamics ISBN 0-9646806-3-7 published in January 2003 with 224 pages and 167 illustrations begins with applicable principles of Newtonian physics explained in aerodynamics context followed by aerodynamics of wings airplanes helicopters and surface effect craft explained in Newtonian context For information and reviews posted on Amazoncom click here Introduction to Aerodynamics list 2950 is available from Amazoncom from Academy of Model Aeronautics AMAMuseum bookstore at Muncie IndianaPhone 765-289-4236 and from DAR Corporation in Lawrence KS phone 785-832-0434 e-mail email protected For further information contact the publisher: Regenerative Press 1900 Romine Road Anderson IN 46011 Discount available for libraries and educational institutions Author Gale Craig is retired from 35 years in General Motors Research and development He holds the MS degree in physics is licensed to teach high school physics and mathematics is named as sole inventor in nineteen US patents in widely varied areas and is a licensed pilot of 1700 hours who owns and flies a Cessna 182 NOTE: The author again presented a one hour forum on this topic during the 2009 Experimental Aircraft Association convention at Oshkosh WI Thursday July 30 at 1:00 PM in the 001 Dake Corporation Pavilion For a copy of the handout go to regenpresscom/handoutpdf Comments or questions cCopyright 2003 2008 Gale M Craig This web page may be reproduced in unbound form for non-profit individual or classroom use if printed in entirety with copyright notice |
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Joint Army Navy NASA Air Force |
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NASA announces plans to send satellites into the Pepsi Galaxy |
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I have been an ESL teacher for almost 10 years My passion for helping others led me to quit my job at NASA and move all the way to South Korea During my time in South Korea I taught thousands of students how to speak English I loved my job and I want to continue helping students just like you enjoy learning English My main goal as a teacher is to help you speak English with confidence Are you ready to learn Well then let’s jump right in! |
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NASA Harvard and the Pentagon Are All Taking UFOs Seriously Now |
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When studying the vegetation reflectance spectrum in detail The near-infrared spectrum itself allows a relatively high detailed probe of the health of plants at a cellular level In photosynthesis the chloroplasts in plant cells take absorb light energy sing chlorophyll which aborbs photons and creates and electronic channel to fix carbon and water to form glucose the basis of carbohydrates and hence food for the plant The forms of chlorophyll in plants absorb light at specific frequencies typically in the red and blue light The green portion of light is effectively reflected this makes the plant was seen in the range of green in the visible spectrum This information is also important for developing lighting systems for indoor agriculture for example of which light frequencies are the most efficient for growing plants as exploited by so-called grow lights for indoor plant growth This is another topic in precision agriculture which can be explored further In any case Chlorophyll pigments A and B absorbs most energy at about 450 nm blue and 650 nm red respectively with significant overlap between the 2 as shown in the diagram Other pigments absorb more visible wavelengths but the most absorption occurs in the red and blue portions of the spectrum This absorption removes these colors from the amount of light that is transmitted and reflected causing the predominant visible color that reaches our eyes as green This is the reason healthy vegetation appears as a dark green Unhealthy vegetation on the other hand will have less chlorophyll and thus will appear brighter visibly since less is absorbed and more is reflected to our eyes This increase in red reflectance along with the green is what causes a general yellow appearance of unhealthy plants Plants reflect strongly in the NIR however not because of chlorophyll but because of a spongy layer of mesophyll tissue found on the bottom surface of the leaf which does not reflect strongly in the red IR reflectance is advantageous to plants as it is reflects electromagnetic energy that the plant cannot use and moreover would probably damage the plant tissues especially during high levels of sunshine where the IR radiation would heat the plant tissues and slow down or damage cellular processes Plant stress causes an increase in visible light transmission in the green as the chlorophyll in the palisade mesophyll decays and the much more obvious effect of an increase in the reflectance of red The near-infrared plateau NIR 700 nm – 1100 nm is a region where reflections are limited to the compounds typically found in the spongy mesophyll cells of leaves made of primarily cellulose lignin and other structural carbohydrates The spongy mesophyll cells located in the interior or back of the leaves reflects NIR light much of which emerges as strong reflection rays The intensity of NIR reflectance is commonly greater than most inorganic materials so vegetation appears bright in NIR wavelengths NIR reflection in the Shortwave Infrared region is also affected by multiple scattering of photons within the leaf related to the internal cellular structure fraction of air spaces in the xylem vessels of the plant and most important as a general indicator the air-water interfaces that refract light within leaves The primary and secondary absorption of water in leaf reflectance are greatest in spectral bands centered at 1450 1940 and 2500 nm with important secondary absorptions at 980 nm and 1240 nm Ref-3 These are the bands which create the primary NIR reflectance in healthy plants The spongy mesophyll tissue itself in plant foliage is critical for gas exchange CO2 for use in photosynthesis and O2 for excretion as a waste product from the photosynthesis process itself spongy mesophyll contains air gaps to allow for the gas exchange and also contains much of the vascular tissue of the leaf with the xylem vessel portion facing into the spongy mesophyll and the phloem tubes facing towards the palisade mesophyll where most photosynthesis occurs The cohesion-adhesion model of water transport in vascular plant tissue describes how hydrogen bonding in water to explain many key components of fluid movement through the plant’s xylem and other vessels Within a vessel water molecules hydrogen bond not only to each other but also to the cellulose chain itself which comprises the wall of plant cells This creates a capillary tube which allows for capillary action to occur since the vessel is relatively small This mechanism allows plants to pull water up into their roots Furthermorehydrogen bonding can create a long chain of water molecules which can overcome the force of gravity and travel up to the high altitudes of leaves Cohesion-adhesion model of water transport in cellulose-based plant vascular tissue Therefore the existing interface between the water in plants and the cellulose chains is a key indicator that a plant is 1 not under dehydration and 2 has structural integrity If any one of these factors are removed this indicates poor plant health and this is detected by the reflectance in the NIR Whenever a plant becomes either dehydrated or sickly the spongy mesophyll layer collapses and this will cause the plant to reflect as less NIR light Furthermore it will interrupt gas exchange thus causing the chloroplast containing palisade cells to die and cause the leaf to become more red Soil on the other hand reflects both NIR and Red Thus a combination approximated as linear of the NIR reflectivity and red reflectivity should provide excellent contrast between plants and soil and between healthy plants and sick plants The Normalized Difference Vegetation Index NDVI is a simple graphical indicator that can be used to analyze remote sensing measurements such as in aerial and space based surveys and assess whether the target being observed contains live green vegetation or not It turns out which combination is not particularly important but the NDVI index of NIR-red/NIRred does happen to be particularly effective at normalizing for different irradiation conditions Hence plants in a given area that are adequately hydrated show high absorbance of NIR light in this absorbance band and low reflectance whereas those subject to drying shows greater reflectance in this band NASA scientist Dr Compton Tucker examined 18 different combinations of NIR Landsat MSS 7 800-1100 nm red Landsat MSS 5 600-700 nm and green Landsat MSS 4 500-600 nm and compared these results with the density of both wet and dry biomass to in an attempt determine which combination correlated best Ref-4 His findings were that NIR/red SQRTNIR/red NIR-red NIR-red/NIRred SQRTNIR-red/NIRred05 all of the above are very similar indicators for estimating the density of photosynthetically active biomass others go further in specific analysis using indices based the slopes of the line going across certain edges found in scatter plots of the NDVI vs RED pixels themselves Others use forms of linear regression support vector machines and trained neural networks to examine the hyperspectral images to look for patterns Typically one can use an experimentally determined threshold level for near IR reflectance from healthy plants to allow for a way to label plant health remotely |
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NASA amp SPACE |
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NASA Reveals Incredible Images of Massive Comet that Flew Past Earth |
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Mikrolar Helps NASA Complete Webb Telescope Center of Curvature Pre-test |
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Ex-NASA physicist takes flight in Chehalem Mountains |
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NASA Visitor Center Locations |
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IHRF Part Of NASA Research Team On Microgravity-Induced IH |
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EmDrive: polémica por el motor de la NASA que violaría la tercera Ley de Newton |
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Note: The Developmental Educators’ Handbook DEH is adapted from a NASA Project conceived in 1989 called the Space Educators’ Handbook SEH The editor of that work was Jerry Woodfill a NASA employee As such the DEH would be deemed a NASA Spinoff as technology spawned by NASA space exploration – finding a useful secondary application in the public sector The official designation for the SEH is OMB/NASA Report No S677 Portions of the DEH are interactive with the SEH The home page for the Space Educators’ Handbook is opened by clicking here Both the SEH and DEH are in the public domain Jerry and Betty Woodfill developed this program known as the DEVELOPMENTAL EDUCATORS’ HANDBOOK for educational use Approval to use copyrighted materials which are not public domain has been obtained except for items which are deemed Fair Use by the copyright act of 1976 Content and materials included in this program do not reflect policy research or endorsement of the authors’/editors’ employers Please contact Mr Woodfill if errors in data spelling or program operation are found |
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NASA 25 Hours of Thunderhill Presented by Hawk Performance |
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Teams Prepare Cars and Drivers for the 2018 NASA 25 Hours |
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NASA Northeast’s Alan Cohen Is Pushing His Cadillac CTS-V to New… |
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Stunning NASA video showcases the beauty of our sun |
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Sonda Parker da NASA tira primeira foto dentro da atmosfera do Sol |
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Kurtka Alpha Ind MA-1 VF59 NASA – Replica Blue |
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Piante che depurano l’aria di casa e ufficio: le migliori secondo la NASA |
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Der Saturn verliert seine Ringe amp die NASA weiß schon wann |
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NASA’s InSight lander just took its first selfie on Mars — |
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NASA kids |
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Sonda NASA New Horizons se pregăteşte de un survol istoric în 1 ianuarie: Ultima Thule un vestigiu de la formarea sistemului solar |
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NASA lança site para compartilhar programas de código aberto |
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NASA’s New Horizons is poised for a historic New Year’s Day flyby – BGR |
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Anh cựu kỹ sư NASA chế bom thiên hà kim tuyến mùi đánh rắm thừa nhận video có một số cảnh dàn dựng |
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NASA Rally Sport – 2017 Excitement! |
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NASA Warns Saturn’s Rings Disappearing: “Worst Case Scenario” |
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¿Qué micrófono llevarías a Marte La NASA ya ha escogido |
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2018 NASA 25 Hours of Thunderhill Presented by Hawk Performance: 6-Hour Update |
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Sixty years of NASA thirty years of NASA leadership |
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Margaret Hamilton NASA’s First Software Handler Comedian |
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Secondo uno studio finanziato dalla NASA siamo vicini ad un crollo di civiltà |
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NASA – APOD |
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La NASA reconoce que por error enviaron la nave inSight a Almería |
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La NASA graba por primera vez el sonido del viento de |
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Elevi ai Colegiului Naţional “Gheorghe Lazăr” din Sibiu calificaţi în faza finală a unui concurs NASA Pentru a merge au nevoie de 22000 de euro |
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CANAL PÚBLICO DE NASA TELEVISIÓN |
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NASA Celebrates Native American Heritage Month |
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NASA Kaçak Karadeliği Araştırıyor! |
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La NASA aseguran que el planeta Saturno está perdiendo sus anillos |
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The Traditional Orange NASA Space Suit Just Got A Makeover |
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The Most Earth Size Habitable Zone Planets around a Single Star on This Week NASA – 02/24/2017 |
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Red Digital Cinema amp NASA: First 8K Video From Space Shot on HELIUM |
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Die NASA Mond- und Marslandungsschwindel |
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Cobham Technology Enables NASA’s InSight Mission to Mars |
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Sonda da NASA Detecta Anomalias Dias Antes da Aproximação de Objeto Extraplanetário |
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La NASA appelle Lapin découvert vie sur Mars par la NASA rover Opportunity 2015 |
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Jurugambar Rakam Klip Bola Api Seperti Ingin ‘Menghentam’ Bumi NASA Beri Penjelasan |
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NASA sửng sốt vì tiểu hành tinh hà mã bay ngang bầu trời |
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NASA sees the light for the future of space communications |
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Dua Bekas Astronaut NASA Silang Pendapat soal Kirim Manusia ke Mars |
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Una colaboración de NASA con Google ha encontrado una estrella distante que está siendo orbitada por ocho planetas al igual que nuestro pr |
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NASA Has Advice for Rescuing Tony Stark from His Fate in the Avengers: Endgame Trailer |
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Νέα κυβερνοεπίθεση στη NASA |
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Former NASA engineer built a glitter bomb and filmed package thieves getting instant justice |
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جارو برقی ۲۵۰۰ وات ناسا مدل NASA NS-9095 |
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NASA FDL 2017 연구 소개 |
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The Origins of NASA’s Mercury Program |
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Afirman que se hackearon servidores de la NASA |
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Alle Spuren führen zur NASA |
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La NASA entrena a una joven de 17 años que se |
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Project Icebridge: Η NASA ανακάλυψε μέσω δορυφόρου πανάρχαιο οικισμό στην Ανταρκτική |
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NASA ANNOUNCED THAT IT COMMUNICATES WITH 4 RACES OF ALIENS |
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Марсианские хроники: как турецкие иммигранты стали подрядчиками NASA – Большие новости |
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Hatalmas méretű fekete korong száguldott át a felvételen a Hold felszíne felett! A NASA titkos archívumába dugták el a felvételt |
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How should NASA return to the Moon |
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Mis à jour : Les extraterrestres pourraient déjà être sur Terre selon un scientifique de la NASA |
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Latest view of Jupiter from NASA’s Juno spacecraft |
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NASA TV Media |
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IHRF Scientific Advisor Awarded NSBRI/NASA Grant to Study Non-Invasive Pressure Monitoring |
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NASA skrivala dokaze: Zemlja nije ni ravna ni okrugla već zaobljena! |
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ATs KEEP NASA ASTRONAUTS SAFE AS THEY TRAIN FOR THEIR MISSION |
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Toyo Tires/Flying Lizard Motorsports Wins Record Fourth 2018 NASA 25 Hours… |
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2018 NASA 25 Hours of Thunderhill Presented by Hawk Performance: 10-Hour… |
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Cosa ha detto realmente il tecnico della NASA sulla possibilità che gli alieni ci abbiano già visitato |
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NASA bën një zbulim të befasishëm në asteroidin që sapo filloi të analizojë |
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El error de código de la NASA que costó 150 millones de dólares |
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Carlos Bacelli teria inventado que Chico Xavier foi examinado pela NASA |
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2018 NASA 25 Hours of Thunderhill Presented by Hawk Performance: 6-Hour |
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NASA at 60-something |
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NASA Advanced Software Development |
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NASA to welcome plans for AI lunar robot |
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From NASA to SpaceX just look to space for good news from 2018 – CNET |
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A simple “Asteroid Detection System” with Lightning Components and a NASA Asteroids NeoWs API |
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На Марсе начал работу научный аппарат NASA InSight / techvestiru |
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Η NASA «αγγίζει» τον ήλιο!! |
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Las Vans Space Voyager en Homenaje a la NASA ya se Venden en Perú |
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Bob Lazar: The truth behind Area 51 and the signal from Mars : NASA reveals having received a mysterious signal of unknown origin from Mars |
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Gigantic ‘Object’ Spotted Towering Over Earth From ISS – Before NASA Live Feed is Cut |
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NASA’s InSight Mission – Exploring the uncharted territory of Mars |
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NASA’s InSight Has a Thermometer for Mars |
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Στον αστεροειδή «Μπενού» έφτασε το σκάφος OSIRIS-REx της NASA |
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NASA’s Environmentally Responsible Aviation Could Help Saving Billions In Commercial Airlines |
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From NASA to SpaceX just look to space for good news from 2018 |
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NASA adverte que as geleiras mais estáveis da Antártida começam a derreter |
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POSSIBLE 2 COQUILLAGES TROUVÉS PAR CURIOSITY NASA SUR MARS |
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From Pittsfield to the red planet: The Berkshire native at the heart of NASA’s InSight Mars mission |
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NASA’s 1st flight to moon Apollo 8 marks 50th anniversary |
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NASA Bombards Antarctic With Latest Satellite |
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NASA divisa un ‘árbol de Navidad’ en algún rincón de la Tierra |
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Κρατήστε την ημερομηνία: 8 Αυγούστου 2023 Γιατί τότε ένας αστεροειδής διπλάσιος από το Άγαλμα της Ελευθερίας ενδέχεται να προσκρούσει στη Γη σύμφωνα με τη NASA |
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Queen guitarist Brian May releases tribute to NASA spacecraft |
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Wallpaper Computer – NASA’s New Spacecraft Will Have A Star Trek-Like Cockpit |
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Flat Earthers claim Australia doesn’t exist and people who live there are actors paid by NASA |
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La NASA pondrá hoy en órbita al satélite TESS gracias al Falcon 9 de Space X |
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NASA halla indicios de Vida Extraterrestre Vídeo en 360° |
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X-WARE Aerospace Defense Avidyne Ball Aerospace Boeing General Dynamics JPL NASA Northrup Grumman Raytheon and more |
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A woman who introduced herself as a NASA employee spoke about the flight of a man on Mars |
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Πρώην μηχανικός της NASA έφτιαξε μια επική εκδίκηση για τους κλέφτες δεμάτων |
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Víz nyomára bukkant a NASA űrszondája a Bennu drónvideó – kardszárnyú delfinek csatlakoz |
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NASA RADARLARI HOLİDAY ASTEROİDİNİ GÖRÜNTÜLEDİ |
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Acusan a Chinos de Hackear la NASA y mas |
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El asteroide Bennu es visitado por OSIRIS-REx una nave espacial de la NASA |
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Statek kosmiczny NASA coraz bliżej tajemniczego obiektu w Pasie Kuipera |
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NASA Proposes Increasing The Fresh Air In The House: Plants |
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NASA Confirmed Data Breach After an Internal Server Was Hacked |
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用NASA科技做菜！安全無毒不沾鍋「鈦」健康！ |
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The How and Why of NASA’s New Super Touring Tire Measurement |
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NASA Released Images Of The Process Of Earth’s Aging |
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NASA की बड़ी कामयाबी: इनसाइट लैंडर ने मंगल ग्रह की |
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MEDIA EVENT FOR THE NEW HORIZONS FLYBY OF ULTIMA THULE Jan 2 2019 2:00 pm ET NASA TV |
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MEDIA EVENTS FOR NEW HORIZONS FLYBY OF ULTIMA THULE Dec 31 2018 2:00 3:00 and 8:00 pm ET NASA TV |
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Flat Earth Man – Puppet Show eine lustige NASA / ISS Entlarvung dt Untertitel |
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All photos by Irem Tumer except comet Lovejoy image which is from NASA |
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10 NASA-approved Air Purifying Houseplants plus 4 other ways to use plants to detox your life |
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MEDIA EVENT FOR THE NEW HORIZONS FLYBY OF ULTIMA THULE Jan 1 2019 11:30 am ET NASA TV |
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니콘 프로페셔널 풀프레임 DSLR 카메라 D5 NASA |
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Un astronauta de la NASA aprende ‘de nuevo’ a caminar VIDEO |
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Jupiter Great Red Spot NASA |
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Closing In On Launch: NASA’s Gold-Mirrored 8 Billion Webb Space Telescope |
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Disfraz de Astronauta de la NASA Infantil |
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Does This NASA Image Show Aliens on Mars |
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NASA: Intelligent Aliens Are On Earth Hiding In Plain Sight Intelligent aliens may have already visited our planet and are currently hiding in plain sight according to NASA |
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Review: NASA Spaceflight: A History of Innovation |
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Solar Eclipse Footaged Leaked Early From NASA |
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Create Mind Blowing Images Of Jupiter Using NASA’s RAW Data From The Juno Mission |
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‘Moon Mission’ NASA Moon landing ‘verify’ to Russia |
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La NASA offrirà centomila dollari di premio per il design di un sensore |
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ក្រុមការងារ NASA វេចខ្ចប់កែវយឺតអវកាស James Webb… |
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NASA radarları Holiday Asteroidini |
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Η NASA τοποθέτησε σεισμογράφο πάνω στην επιφάνεια του Άρη |
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NASA Engineering and IT Integrative Support |
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NASA Videos: |
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NASA que hace una estructura circular en Marte |
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What robots in space teach us about teamwork: A deep dive into NASA |
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Bio NASA Thermal Mask – Meteorite Powder |
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Space Shuttle Controls Necktie NASA Tie |
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Update on NASA Mission to Touch the Sun by Professor MK… |
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Mời tải về hình nền iPhone XS dựa trên ảnh chụp Sao Mộc của NASA |
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CARA TINGKATKAN PRODUKTIVITAS BUDIDAYA PERIKANAN DENGAN NUTRISI ORGANIK NASA |
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So genial wirbt die NASA für Expeditionen ins Weltall |
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Дмитрий Маликов стал двоюродным корабль NASA приближается к загадочному Ультима Туле |
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La NASA ha pubblicato i suoni di Marte registrati dalla sonda Insight |
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NASA Is Hiding Mysterious Stream Of Smoke Fro |
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NASA findet Mondflug-Wissen nicht wieder |
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NASA What’s Up — декабрь 2017 |
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NASA Has Released A Photograph Of A Mysterious Spaceship That Crash-landed In Utah |
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Palani student’s artwork to feature in NASA calendar |
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NASA’s InSight sends its first photos back from the surface of the planet Mars |
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A NASA a földönkívüli élet bejelentésének küszöbén áll: “csak egy lépésre vannak attól hogy bejelentsék amit 60 évig már nincs hamarosan pedig a természetgyógyászat is be lesz tiltva! – VIDEÓ |
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January 1 NASA probe will visit one of the most remote objects of the solar system |
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NASA Space Settlement Contest – 2018 |
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Zonal – NASA 2015 |
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The future or lack thereof of NASA’s Mars Exploration Program |
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Impresionante: la NASA mostró a Júpiter como nunca antes |
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NASA Pantau Gunung Anak Krakatau yang Diduga Picu Tsunami Anyer |
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2540 Amazing Photographs Of Mars Has Been Released By NASA |
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Sonda da NASA detecta anomalias dias antes da aproximação de objeto extraplanetário |
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NASA Confirms the Formation of Magnetic Field due to Dynamo at |
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NASA’S NEWLY ARRIVED OSIRIS-REX SPACECRAFT ALREADY DISCOVERS WATER ON ASTEROID |
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La misión InSight de la NASA llega a Marte |
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NASA InSight Mission to Mars Arrives at Launch Site |
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Another view on the problems facing NASA’s Mars Exploration Program |
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NASA as a brand |
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NASA / |
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NASA Astronauts Will Still Ride Russian Rockets After US Craft Arrive |
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It’s Official NASA’s Voyager 2 Spacecraft Leaves the Solar System |
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NASA’s concept Europa lander belongs on the cover of a sci-fi pulp |
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Netflix announces premiere date for true crime docuseries “The Innocent Man” based on John Grisham Neil Armstrong documentary will feature rare home videos and NASA footage |
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JUNIO 21: AÑO NUEVO NASA Y YANAKUNA |
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NASA astronauts will still be riding Russian rockets after the American craft arrives |
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PRODUK SARANA PERTANIAN PETERNAKAN PERIKANAN PERKEBUNAN DAN KEHUTANAN BERBAHAN ORGANIK BUKTIKAN SEGERA KEUNGGULAN PRODUK NASA DAN JADILAH DISTRIBUTOR NASA DI DAERAH ANDA |
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ODG и NASA разработают умные 3D AR-очки для астронавтов |
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NASA potvrdila: Marihuana obsahuje mimozemskou DNA která nepochází ze Sluneční soustavy |
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Inicio del programa estudiantil internacional NASA |
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MEDIA EVENT FOR THE NEW HORIZONS FLYBY OF ULTIMA THULE Jan 3 2019 2:00 pm ET NASA TV |
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NASA Speed News Magazine |
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NASA Your Library |
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70 塊立方體完美貼合的 360 NASA 1152％ ｜ 全球狂銷 5000 顆好眠枕 ｜ 募資金額突破千萬台幣 |
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Grid Locked in as Teams Ready for Saturday’s 2018 NASA 25 Hours of Thunderhill Presented by Hawk Performance |
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2018 NASA 25 Hours of Thunderhill Presented by Hawk Performance: 6-Hour… |
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Experto de la NASA habla sobre Chemtrails y los riesgos de la Geoingeniería |
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NASA Worldview |
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ក្រុមការងារ NASA វេចខ្ចប់កែវយឺតអវកាស James Webb ដោយប្រយ័ត្នប្រយែងបំផុត |
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The Bridenstine era begins at NASA |
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Hidden No More: Street Outside Of NASA’s Headquarters To Be Renamed ‘Hidden Figures Way’ |
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NASA : Ο Κρόνος χωρίς δακτύλιους |
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La NASA manda a una limpiadora a la Estación Espacial Internacional |
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NASA’s InSight places first instrument on Mars |
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Karamoko Dembele puştiul de senzaţie de la Celtic Glasgow la doar 15 ani a semnat primul contract profesionist cu pe hoţi poate fi frumoasă şi chiar festivă! Un inginer NASA a creat o bombă cu sclipici |
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Mars’ın Sesi İlk Defa NASA’nın Paylaştığı Kayıt İle Duyuldu |
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2018 NASA 25 Hours of Thunderhill Presented by Hawk Performance: 2 |
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Brian May estrenará nueva canción desde la NASA |
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NASA впервые удалось записать шум ветра на Марсе |
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A Való Világ ehhez képest egy NASA konferencia: íme minden idők legcikibb kv… |
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Putting astrobiology at the heart of NASA science |
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Ultima Thule: El lejano y misterioso objeto celestial al que llegará una sonda de la NASA |
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NASA Finds Long-Lost Opportunity Rover Months After it Lost Communication |
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Elon Musk Smoking Weed NASA Investigation SpaceX Project |
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NASA Inlay Sweater repblue – 20 OFF |
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Les fans harcèlent la NASA pour sauver Tony Stark |
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New NASA lander captures 1st sounds of Martian wind |
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NASA створює постери до всіх своїх місій у стилі коміксів та серіалів |
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NASA facility in Fairmont renamed after mathematician Johnson |
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Los anillos de Saturno están colapsando advierte NASA |
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ASTRONOMÍA PARA ESTE MES POR NASA / JPL JET PROPULSIÓN LABORATORY |
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Why Should You Join NASA |
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1958 NASA exploring space |
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NASA Marine |
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display • Realme will be the first to offer MediaTek Helio P70-powered smartphones • Dell Latitude 7390 and 7490 are getting new aluminum chassis options • Asus now shipping ZenBook UX333/433/533 series with narrow bezels on all four sides • New ‘Oppo R19’ renders point to dual in-display selfies this time • Apple is losing its European market share to Xiaomi and Huawei – ZyrnoMarket • Apple Samsung Oppo HTC ZTE Infinix Tecno Huawei and itel Products dominate the Smartphone Market in Nigeria – Zyrnosoft Media • Pictures of the Sapphire Radeon RX 590 Nitro Special Edition surfaces online • LG patents a true bezel-less smartphone display with an in-display selfie camera • Images and video of a new Asus ZenFone 6 prototype device leaks online • The LG G7 Fit is to be released into new markets soon • Mysterious Macbook Air-like device with i7 processor shows up on Geekbench • The MacBook Air Retina is slightly more repairable than Apple’s other laptops • Panasonic announces enhancements 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Successor Byte App to Launch Spring 2019 Creator Confirms • Nokia 81 Spotted in List of ARCore Supported Devices Launch Appears Imminent • Chrome 71 to Start Showing Warning Alerts on Unclear Billing Websites • YouTube Nintendo Switch App Available for Download Now • MacBook Air 2018 Has a Bunch of Modular Components Ingress-Resisting Keyboard – iFixit • Google Says Dark Mode Can Help Improve Battery Life on Android Devices • HTC Vive Focus Now Available in Global Markets Other Enterprise Focused Tools Also Unveiled • Files Go App Officially Rebranded to Files Used by Over 30 Million Monthly Active Users • Facebook Brings Dating Service Test to Canada Thailand • WhatsApp for iOS Beta Version Now Publicly Available • OnePlus 6T Teardown Reveals Camera Lens in Optical In-Display Fingerprint Sensor • Amazon Cisco Partner to Help Businesses Run Apps in the Cloud • iPhone XR May Get Improved Haptic Touch Feature to Expand Notifications With iOS 1211 • Samsung Galaxy S8 Galaxy S8 and Galaxy 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Вече официално от NASA: EmDrive работи |
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Εξωγήινοι ζουν ανάμεσα μας Ερευνητής της NASA ρίχνει «βόμβα μεγατόνων» και «γκρεμίζει» το ίντερνετ |
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UFO Expert Says NASA Will Discover Alien Life After About 5 Years |
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America the Beautiful at Night – Amazing Photographs of America’s Great National Parks amp Landmarks – by Wally Pacholka – 3x Time-Life Picture of the Year – Published by NASA APOD 45x – Please hover over the National Parks button above to view Wally Pacholka’s Photographs |
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Essity awarded contract by NASA to Develop Next-Generation Compression Garment for Orion Missions |
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PHYSICAL PRINCIPLES OF WINGED FLIGHT c Copyright 2003 2008 Gale M Craig For a pdf version of this site click here PLAUSIBLE FALSEHOOD Popular explanations of how wings work are often erroneous and scientifically unsound Wrong explanations may be given by well-meaning teachers and others but false teaching may sometimes be just for convenience Many years ago a famous aerodynamicist Dr Theodore Von Karman instructed his assistant: When you are talking to technically illiterate people you must resort to the plausible falsehood instead of the difficult truth From Stories of a 20th Century Life ISBN 0-915760-04-5 by WR Sears former assistant to Von Karman Falsehood whether intentional or not is still being taught The most popular theory of wing operation which we may call Hump Theory because it requires a wing to have a more convex upper surface as compared to the lower is easily shown to be false Hump theory is based on an assumption of equal transit times that air passage over a curved upper wing surface must occur in the same length of time as air passage below where the surface is more flat and hence of a shorter path length In order to have the same transit time flow at the longer path upper surface must be of greater velocity than that at the lower surface Thus in accordance with Bernoulli’s law it is reasoned that upper surface pressure must then be less than at the lower surface thereby producing upward lift Equal transit time is sometimes illustrated by representing bits of passing flow above and below an airfoil or wing as shown here: Although Bernoulli’s law is sound and well proven this popular explanation world-wide of wing operation is false Upper surface flow is indeed faster than the lower so much so that upper surface transit time is normally less than the lower as indicated here: Although the assumption of equal transit time is wrong and has no basis in known physics it can be found in books from otherwise reputable publishers such as National Geographic Macmillan and others in this country and abroad College level teaching of aerodynamicists and aeronautical engineers does not include equal transit time which cannot survive mathematical investigation The fallacy of equal transit time can be deduced from consideration of a flat plate which will indeed produce lift as anyone who has handled a sheet of plywood in the wind can testify As indicated in the above figure air approaching the plate such as a sheet of plywood accelerates into the reduced above-plate pressure with increasing velocity while air approaching below is slowed in the increased pressure in accordance with Bernoulli Thus faster upper surface flow can be described as a result of pressure difference rather than the cause of it Bernoulli’s principle law or effect states that velocity varies in inverse relation to pressure but does not assign cause-and-effect relation Unfortunately a great amount of confusion has been generated by abuse of Bernoulli’s law in erroneous cause-and-effect explanations PHYSICAL REALITY Basic Newtonian principles of aerodynamic lift and propulsion include: 1 Upward lift is derived by accelerating air mass downward 2 Forward propulsion of propellers and jets is gained by accelerating air mass rearward 3 Drag is incurred through accelerating air mass forward as by viscous coupling For the most part this is undesirable but unavoidable 4 Air mass recirculates upward at a rate equal to the rate at which it is displaced downward in the lift process thus normal atmospheric mass distribution is maintained Sir Isaac Newton 1642-1727 provided for us laws of physics which govern aerodynamic lift His first law is that velocity of an object or bit of mass changes only when the mass is acted upon by applied force The second law states that when force is applied to a mass it accelerates meaning changes velocity at a rate equal to the force-to-mass ratio The term velocity includes both speed and direction Thus force is required to change direction or speed of an object We drive our cars by operating a system of accelerators These include throttle brakes and steering wheel as indicated here Newton’s third law states that mass resists acceleration with equal and opposite force Thus when a cannon accelerates a missile forward the cannon recoils in rearward direction in response to the opposite force In air or other fluid force is exerted in pressure difference Pressure difference between two positions in air is distributed across the distance between positions as a pressure gradient equal in magnitude at any point to the ratio of pressure difference increment dp to distance increment ds at that point Direction of the pressure gradient is toward the position of greater pressure The direction of air acceleration is toward lower pressure and thus opposite to the direction of pressure gradient A tornado has reduced air pressure at its center Thus air moving around it is accelerated toward reduced center pressure centripetal acceleration so that it continuously curves around the center In opposition with equal counter-centripetal force air pushes outward from the center producing an outward pressure gradient which we may refer to as a centrifugal pressure gradient In a demonstration sometimes wrongly described as showing lift due to pressure reduction in moving air or pressure reduction due to flow path restriction a ball or balloon is suspended by a jet of air Such demonstration may be designed for the purpose of attracting attention to the vacuum cleaner section of a department store In reality flow follows the surface of the ball in a phenomenon known as surface attachment or Coanda effect producing in the curving flow an outward pressure gradient which reduces surface pressure so that the ball is drawn into the flow Coanda effect can be demonstrated by the device shown in the following photograph In demonstration the coffee can is removed and the candle lighted The small blower is then energized blowing air above the candle with no significant effect on the flame The blower is then stopped and the coffee can is inserted as shown When the blower is again energized flow follows the can surface downward and extinguishes the candle flame The blower shown was found in an electronics flea market Perhaps a small hair dryer operating at low speed would suffice for one wishing to produce a similar demonstration A popular classroom demonstration of aerodynamic lift involves a drooping sheet of paper as in figure A which lifts when air is blown over it This is often mistakenly described as pressure reduction in moving air according to to Bernoulli principle In reality it is another demonstration of pressure reduction in a centrifugal pressure gradient The upward pressure gradient in downwardly accelerating flow opposes atmospheric pressure resulting in upper surface pressure reduction and lift If the lift in figure A were caused by Bernoulli principle then the paper in figure B should droop further when air is blown beneath it However as shown it raises when the upward pressure gradient in downward-curving flow adds to atmospheric pressure at the paper lower surface Curvature of flow and resulting pressure gradients is the source of lift for heavier than air aerodynamic flight Description of lift as due to pressure gradients in curvature of above-wing and below-wing flows was given by Otto Lilienthal in his 1889 book titled Birdflight as the Basis of Aviation Unfortunately that has been neglected for well over a century in favor of pseudoscientific descriptions Lilienthal found that wing curvature from leading edge to trailing edge improved efficiency by adapting the wing to required curvature of flow CIRCULATION Higher level theory involves Bernoulli in lift explanation but attributes difference in velocity between above-wing and below-wing flows to circulation rather than equal transit time All air movements are circulatory Circulation begins to develop when an airfoil is started into forward motion As indicated in the next figure air following the surfaces is accelerated downward Because of downward acceleration pressure below the airfoil is increased and pressure above it is decreased In beginning response to the pressure difference upward recirculation of air mass occurs around the leading and trailing edges As forward airfoil movement continues downward movement left behind the trailing edge divides into forward and rearward rotational patterns carrying upward recirculation around the aifoil and around the center of a receding aft vortex created in shear between downward and upward movements Forward recirculation around the airfoil is known as circulation while the recirculation pattern left behind is known as the starting vortex The principle of conservation of angular momentum demands angular momenta of the two rotational patterns be equal and opposite In equivalent classical teaching Helmholtz’ vortex theorems require equal and opposite vorticity Circulation is regenerative Pressure difference produces circulation and as circulation upward momentum ahead is intercepted by wing forward movement and recurved downward more pressure difference is produced Thus circulation increases regeneratively until reaching the limit at which it provides downward movement at the rear for flow to depart the trailing edge in the pointed direction Circulation in excess of this would be opposed by airfoil direction With mature circulation providing downward movement matching the need for flow departure in the pointed direction at the trailing edge participation of the starting vortex is no longer needed or accepted and growths of starting vortex and forward circulation cease In this mature and stable circulation state called the Kutta condition theoretical lossless lift of an airfoil is equal to the rate of circulation downward momentum produced plus an equal rate of upward recirculation momentum intercepted by airfoil forward movement For derivation of the equation for ideal lift without losses in two-dimensional flow click here Evidence of the regenerative nature of circulation and lift can be sometimes be seen in flow downstream from bridge pilings A round piling has no departing flow direction limiting feature This allows circulation and lift to increase until stall occurs which is followed by lift reversal due to effects of residual boundary layer and pressures A trail of alternating vortices occurs as circulation and lift periodically produce stall and reversal In each reversal of circulation and lift a new starting vortex is carried downstream For more on regeneration of circulation of round cylinders click here Circulation is sometimes described as a rotational movement added to or superimposed on passing flow With the added rotational component oncoming flow can be described as rising upward ahead of an airfoil or wing descending behind having increased velocity above and decreased velociy below Alternatively circulation can be considered as a transitory rotation of air which travels with the airfoil or wing through otherwise relatively still air in a manner somewhat analogous to movement of water as it is parted laterally by a ship bow and rejoins at the stern in semicircular-like movements Such water movements travel with the ship even though the water does not The concept of superimposed circulation may seem a bit abstract but circulation is a real rotational movement that travels through the air with the wings of an airplane in flight Alternate perspectives of fixed wing in passing flow and moving wing through still air are illustrated below In passing flow perspective flow curves upward ahead of the lifting wing or airfoil toward reduced pressure above and away from increased pressure below accelerates rearward into reduced above-wing pressure with increased rearward velocity and decreases rearward velocity below in response to increased pressure At the trailing edge upper and lower flows merge to leave in downward direction pointed by the airfoil Merging into common direction and pressure upper and lower flow velocities difference ceases to exist Downward departing flow curves upward toward being parallel to the path of flight again in response to gradients of decreased above-wing pressure and increased below-wing pressure In stable flight circulation kinetic energy is recovered from flow left behind and imparted to oncoming flow ahead through fore and aft pressure gradient couples and Bernoullian process In lossless stable two-dimensional lift condition equal opposing forward and rearward couples have no net effect on circulation angular momentum which remains constant If angle of attack is increased then downwash demand is increased for re-establishing the Kutta condition Increase is momentarily shared between circulation increase and production of a new starting vortex Angular momentum increase in wing circulation is matched by equal and opposite angular momentum production in the new vortex When the Kutta condition is again established growths of starting vortex and circulation cease and the starting vortex no longer involved with circulation is left downstream In the opposite case of decreased angle of attack a portion of airfoil circulation in excess of that permitted by the Kutta condition is deflected by the airfoil and carried downstream as a new vortex of angular momentum equal in magnitude to that of circulation reduction and of same direction as circulation In two-dimensional flow of wind-tunnel wall-to-wall confinement of an airfoil section circulation and starting vortex develop as two opposite-direction vortices formed in division of downwardly driven air into forward and rearward paths of upward recirculation In this case the stabilized rate of upward recirculation ahead is equal to the rate of downward displacement left behind In the case of a real wing beginning lift in open air where there is no barrier to lateral loss around wing ends circulation starting vortex and wing end vortices are all part of a closed loop of recirculation vorticity as indicated below formed around downwardly accelerated air behind a wing Conservation of angular momentum is maintained as angular momentum on one side of the vortex loop is matched by equal and opposite direction angular momentum on the other side For our purposes air driven downward by a wing or airfoil will be termed downwash and upward recirculation will be termed upwash For a real wing in stable flight upwash is driven upward ahead of the wing and around wing ends by pressure difference between above-wing and below-wing regions Upwash around wing ends in combination with downwash left behind produces twin vortices which trail behind wing ends The starting vortex portion of the initial loop is left behind at the beginning of trailing vortices and has virtually no lasting involvement in wing performanceContinuous production of trailing vortices involves continuous energy loss A helicopter which also derives lift from producing downwash leaves a similar pair of trailing vortices behind but helicopters can fly much slower than fixed-wing craft of comparable weight and thus can leave much more intense vortices The following picture courtesy of NASA shows a vertical column of red smoke drawn into a recirculating downwash vortex left behind a slow flying fixed-wing cropduster If circulation changes as necessary to accommodate changing lift conditions strength of trailing vortices equally changes and a new starting vortex is left behind equal and opposite in angular momentum to change of angular momentum in wing circulation Thus conservation of angular momentum is maintained The body of downward air propulsion and upward recirculation is extensive involving a large rate of air mass in wing circulation and trailing vortices Inertial opposition of the large rate of air mass involvement to downward acceleration allows the required rate of downward air mass acceleration required for lift to be achieved with a sufficiently low effective downward velocity that the rate of kinetic energy input proportional to mass times velocity squared production is small enough for practical flight In level flight the downward momentum produced in lift is ultimately intercepted by the Earth’s surface thereby transferring airplane weight to the surface However a steeply banked turn may produce mostly horizontal momentum which would be retained in continued atmospheric movement That wing lift can be effective in maneuvers of any attitude demonstrates the fact that lift at altitude is in reaction to acceleration in a large mass of circulation not necessarily dependent on weight force transfer to the ground Pressure gradients which produce rising circulation ahead of a wing also produce upward recirculation around wing ends leaving trailing vortices behind In wind tunnel two-dimensional flow airfoil tests trailing vortices are totally prevented by confinement of tunnel walls For a wing operating in open air limited confinement is effected by inertial resistance of surrounding air mass to acceleration Longer wingspan produces greater lateral path length with greater mass inertial resistance to lateral acceleration and associated performance loss The following figure indicates relative lift performances of equal area wings with differing span Differing span is represented in differing aspect ratio the ratio of span to chord INDUCED DRAG The lift process also produces forward and rearward thrust components as the force vectors here indicate Forward thrust results from leading edge pressure reduction in the rearward curvature of rising circulation ahead This pressure reduction known as leading edge suction varies with the ratio of lift to airspeed and on some airplanes is piped from a leading edge port as shown below to a stall warning horn in the cabin Another type of stall sensor actuates when intense circulation around the wing leading edge lifts a small vane also shown which operates a switch connected to an electrical warning horn In reaction to downward and forward acceleration in redirection of circulation at the rear of a wing lift and rearward thrust are generated If there were no losses the rearward thrust would be equal to the forward thrust generated in leading edge suction associated with circulation However the rate of rising air mass ahead of a wing is less than that of descending air mass behind because of partial upward circulation diversion into lateral loss paths around wing ends Thus in addition to loss of lift lateral loss around wing ends causes drag known as induced drag due to rearward thrust of aft circulation redirection being greater than the forward thrust Longer wing spans which provide greater air mass inertial opposition to lateral loss are aerodynamically desirable but limiting factors are stress due to bending moments and space requirements for ground operations and hangaring For special airplanes like Voyager which flew around the world non-stop and unrefueled and the U2 spyplane which flew long high altitude missions long spans were vital Circulation theory is traditionally presented in a highly mathematical context for aerodynamicists and aeronautical engineers as appropriate for aircraft engineering Mathematical complexity and departure from Newtonian logic reduces the utility of most circulation theory books for individuals seeking basic conceptual understanding of wings and aircraft configuration When it was found that mathematical expression of vortex flow velocity involved the same equation as that expressing magnetic field strength induced by electrical current trailing vortices behind wing ends became described as induced The physical basis for induced flow velocity is elusive and authors have written different concepts of it based on differing interpretations of a statement of Ludwig Prandtl 1875-1953 Prandtl stated: The wingtip vortices cause a downward motion of air at the wing which will be shown to be responsible for the drag From Prandtl-Tietjens APPLIED HYDRO-AND AEROMECHANICS ISBN 0-486-60375-X Clark Millikan in Aerodynamics of the Airplane Wiley and Sons 1941 states: The numerical computations in Prandtl’s theory are identical with calculations developed long ago in the theory of electromagnetic induction so that the adjective induced has been taken from the electrical field and introduced into aerodynamics John D Anderson Jr in Fundamentals of Aerodynamics 2nd edition 1991 ISBN 0-07-001679-8 says of trailing vortices: The two vortices tend to drag the surrounding air with them and this secondary movement induces a small velocity component in the downward direction at the wing Krisnamurty Karamcheti in Principles of Ideal Fluid Aerodynamics 1996 ISBN 0-089874-113-0 writes: It is customary to refer to the velocity at any point in the vortex flow as the velocity induced by the vortex It must be understood that this is simply a matter of convenience and does not mean that the vortex is actually causing the flow for they just coexist Clearly there is no common understanding of what induction or induced really means in aerodynamics context Reasonably that is because the concept is at best an analogy TILT OF LIFT DIRECTION In classical aerodynamics which regards trailing vortices as induced from central cores rather than caused by lateral loss of circulation upward movement addition of induced downwash to flow is said to cause downward tilt of flow passing the wing With lift being aerodynamic force perpendicular to relative flow downward tilt of passing flow can account for a rearward tilt of lift illustrated symbolically below The rearward component of tilted lift appears as drag known as induced drag because it is said to be caused by induction effects The concept of drag due to downward tilt of passing flow has merit but the tilt can better be described not as due to addition of induced downwash but due to loss of circulation upwash ahead of the wing as it partially diverts into lateral paths around wing ends making the upwash arrival angle at the wing leading edge less steep than the downwash angle at the trailing edge A MODEST PROPOSAL Obfuscation might have been avoided if the electromagnetism analogy of induction not been so convenient as a substitute for Newtonian reasoning Trailing vortices as well as other aspects of aerodynamic lift should be accounted for in Newtonian terms as proposed by Dr Jaako Hoffren of Helsinki University of Technology in paper AIAA 2001-0872 Quest for an Improved Explanation of Lift presented at the Aerospace Sciences Meeting amp Exhibit in Reno in 2001 If aerodynamics were commonly described in Newtonian terms omitting induction and regarding all air mass accelerations as associated with forces of pressure difference then high school and early college physics and mathematics courses might be enriched with basic aerodynamics topics and false teaching of equal transit time half-venturi theory and electromagnetism-like induction might be abandoned Another questionable point in classical teaching is that of describing circulation as a reaction to the starting vortex which is usually said to be caused by viscous drag at the wing trailing edge The reasoning involves theorems of vorticity which require that creation of any vortex be accompanied by other opposite and equal vorticity This is equivalent to the Newtonian principle of conservation of angular momentum This requirement does not justify assumption that one vortex causes another of opposite rotation as indicated by some authors Starting vortex and circulation can readily be explained as created simultaneously in equal and opposite reactions to pressure gradients occurring when air mass is displaced by airfoil movement Two books by this author present basic aerodynamics in terms of Newtonian principles as taught in high schools and college with circulation but with less mathematical complexity than usually associated with circulation teaching The first soft cover Stop Abusing Bernoulli!- How Airplanes Really Fly ISBN 0-9646806-2-9 with 160 pages and 100 illustrations as shown on the left was published in 1999 The book has sold to many countries around the world and as shown on the right has been translated into Korean by Dr SK Lee for use in Pusan National University Peter Garrison’s review of Stop Abusing Bernoulli! in July 1999 FLYING MAGAZINE recommends it to all pilots and would-be pilots However it is no longer in print although a few copies are still on the shelf at Academy of Model aeronautics AMA museum phone 765-289-4236 A second book with more detail better illustrations and hardcover incorporates the Stop Abusing Bernoulli information and more The second book Introduction to Aerodynamics ISBN 0-9646806-3-7 published in January 2003 with 224 pages and 167 illustrations begins with applicable principles of Newtonian physics explained in aerodynamics context followed by aerodynamics of wings airplanes helicopters and surface effect craft explained in Newtonian context For information and reviews posted on Amazoncom click here Introduction to Aerodynamics list 2950 is available from Amazoncom from Academy of Model Aeronautics AMAMuseum bookstore at Muncie IndianaPhone 765-289-4236 and from DAR Corporation in Lawrence KS phone 785-832-0434 e-mail email protected For further information contact the publisher: Regenerative Press 1900 Romine Road Anderson IN 46011 Discount available for libraries and educational institutions Author Gale Craig is retired from 35 years in General Motors Research and development He holds the MS degree in physics is licensed to teach high school physics and mathematics is named as sole inventor in nineteen US patents in widely varied areas and is a licensed pilot of 1700 hours who owns and flies a Cessna 182 NOTE: The author again presented a one hour forum on this topic during the 2009 Experimental Aircraft Association convention at Oshkosh WI Thursday July 30 at 1:00 PM in the 001 Dake Corporation Pavilion For a copy of the handout go to regenpresscom/handoutpdf Comments or questions cCopyright 2003 2008 Gale M Craig This web page may be reproduced in unbound form for non-profit individual or classroom use if printed in entirety with copyright notice |
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NASA National News |
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NASA’nın Uzay Yemekleri |
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The real surface of Mars Recorded by NASA’s Curiosity rover Now in your 01 |
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Mars Rover Curiosity Lands and NASA Becomes Globa |
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El Mars InSight de NASA ya vuela rumbo a Marte |
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La NASA fue víctima de un ciberataque |
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NASA’s Spacecraft Sent to Touch the Sun Snaps First Image from Inside Sun’s Atmosphere |
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NASA объявит первые экипажи космических аппаратов SpaceX Crew Dragon и Boeing… |
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Двух граждан Китая обвиняют в кибератаках на IBM NASA и другие компании-гиганты 367 |
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NASA wê nêzî cismekî dema destpêka Pergala Rojê bibe |
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В NASA намерены продолжать отправлять астронавтов к МКС на Союзах после 2019 года |
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Лучшие изображения NASA за 2011–2013 года 47 фото |
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Endorsing openness for NASA astronauts |
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NASA’s Super Guppy at Mesa Gateway Airport April 18 2018 |
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Tom Sachs Releasing Limited Edition Chairs Used by NASA |
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This graph based on the comparison of atmospheric samples contained in ice cores and more recent direct measurements provides evidence that atmospheric CO2 has increased since the Industrial Revolution Source: NOAAThe Earth’s climate has changed throughout history Just in the last 650000 years there have been seven cycles of glacial advance and retreat with the abrupt end of the last ice age about 7000 years ago marking the beginning of the modern climate era — and of human civilization Most of these climate changes are attributed to very small variations in Earth’s orbit that change the amount of solar energy our planet receives Scientific evidence for warming of the climate system is unequivocal – Intergovernmental Panel on Climate Change The current warming trend is of particular significance because most of it is very likely human-induced and proceeding at a rate that is unprecedented in the past 1300 years1Earth-orbiting satellites and other technological advances have enabled scientists to see the big picture collecting many different types of information about our planet and its climate on a global scale Studying these climate data collected over many years reveal the signals of a changing climateCertain facts about Earth’s climate are not in dispute: The heat-trapping nature of carbon dioxide and other gases was demonstrated in the mid-19th century2 Their ability to affect the transfer of infrared energy through the atmosphere is the scientific basis of many JPL-designed instruments such as AIRS Increased levels of greenhouse gases must cause the Earth to warm in response Ice cores drawn from Greenland Antarctica and tropical mountain glaciers show that the Earth’s climate responds to changes in solar output in the Earth’s orbit and in greenhouse gas levels They also show that in the past large changes in climate have happened very quickly geologically-speaking: in tens of years not in millions or even thousands3 The evidence for rapid climate change is compelling: Republic of Maldives: Vulnerable to sea level rise Sea level riseGlobal sea level rose about 17 centimeters 67 inches in the last century The rate in the last decade however is nearly double that of the last century4 Global temperature riseAll three major global surface temperature reconstructions show that Earth has warmed since 1880 5 Most of this warming has occurred since the 1970s with the 20 warmest years having occurred since 1981 and with all 10 of the warmest years occurring in the past 12 years 6 Even though the 2000s witnessed a solar output decline resulting in an unusually deep solar minimum in 2007-2009 surface temperatures continue to increase 7 Warming oceansThe oceans have absorbed much of this increased heat with the top 700 meters about 2300 feet of ocean showing warming of 0302 degrees Fahrenheit since 19698 Flowing meltwater from the Greenland ice sheet Shrinking ice sheetsThe Greenland and Antarctic ice sheets have decreased in mass Data from NASA’s Gravity Recovery and Climate Experiment show Greenland lost 150 to 250 cubic kilometers 36 to 60 cubic miles of ice per year between 2002 and 2006 while Antarctica lost about 152 cubic kilometers 36 cubic miles of ice between 2002 and 2005 Visualization of the 2007 Arctic sea ice minimum Declining Arctic sea iceBoth the extent and thickness of Arctic sea ice has declined rapidly over the last several decades 9 The disappearing snowcap of Mount Kilimanjaro from space Glacial retreatGlaciers are retreating almost everywhere around the world — including in the Alps Himalayas Andes Rockies Alaska and Africa10 Extreme events The number of record high temperature events in the United States has been increasing while the number of record low temperature events has been decreasing since 1950 The US has also witnessed increasing numbers of intense rainfall events 11 Ocean acidificationSince the beginning of the Industrial Revolution the acidity of surface ocean waters has increased by about 30 percent1213 This increase is the result of humans emitting more carbon dioxide into the atmosphere and hence more being absorbed into the oceans The amount of carbon dioxide absorbed by the upper layer of the oceans is increasing by about 2 billion tons per year1415 This resource has been selected for inclusion in the CLEAN collection References1 IPCC Fourth Assessment Report Summary for Policymakers p 5BD Santer etal “A search for human influences on the thermal structure of the atmosphere” Nature vol 382 4 July 1996 39-46Gabriele C Hegerl “Detecting Greenhouse-Gas-Induced Climate Change with an Optimal Fingerprint Method” Journal of Climate v 9 October 1996 2281-2306V Ramaswamy etal “Anthropogenic and Natural Influences in the Evolution of Lower Stratospheric Cooling” Science 311 24 February 2006 1138-1141BD Santer etal “Contributions of Anthropogenic and Natural Forcing to Recent Tropopause Height Changes” Science vol 301 25 July 2003 479-4832 In the 1860s physicist John Tyndall recognized the Earth’s natural greenhouse effect and suggested that slight changes in the atmospheric composition could bring about climatic variations In 1896 a seminal paper by Swedish scientist Svante Arrhenius first speculated that changes in the levels of carbon dioxide in the atmosphere could substantially alter the surface temperature through the greenhouse effect3 National Research Council NRC 2006 Surface Temperature Reconstructions For the Last 2000 Years National Academy Press Washington DC4 Church J A and NJ White 2006 A 20th century acceleration in global sea level rise Geophysical Research Letters 33 L01602 global sea level estimate described in this work can be downloadedfrom the CSIRO TC Peterson etal State of the Climate in 2008 Special Supplement to the Bulletin of the American Meteorological Society v 90 no 8 August 2009 pp S17-S187 I Allison etal The Copenhagen Diagnosis: Updating the World on the Latest Climate Science UNSW Climate Change Research Center Sydney Australia 2009 p 01aprdeepsolarminimumhtm8 Levitus et al Global ocean heat content 1955–2008 in light of recently revealed instrumentation problems Geophys Res Lett 36 L07608 20099 L Polyak etal “History of Sea Ice in the Arctic” in Past Climate Variability and Change in the Arctic and at High Latitudes US Geological Survey Climate Change Science Program Synthesis and Assessment Product 12 January 2009 chapter 7R Kwok and D A Rothrock “Decline in Arctic sea ice thickness from submarine and ICESAT records: 1958-2008” Geophysical Research Letters v 36 paper no L15501 National Snow and Ice Data CenterWorld Glacier Monitoring Service11 Note: The pH of surface ocean waters has fallen by 01 pH units Since the pH scale is logarithmic this change represents approximately a 30 percent increase in acidity13 C L Sabine etal “The Oceanic Sink for Anthropogenic CO2” Science vol 305 16 July 2004 367-37115 Copenhagen Diagnosis p 36 |
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LES ANCIENNES RUINES D’UNE ALIEN CITY SUR MARS DANS LES IMAGES OFFICIELLES DE LA NASA |
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Popular explanations of how wings work are often erroneous and scientifically unsound Wrong explanations may be given by well-meaning teachers and others but false teaching may sometimes be just for convenience Many years ago a famous aerodynamicist Dr Theodore Von Karman instructed his assistant: When you are talking to technically illiterate people you must resort to the plausible falsehood instead of the difficult truth From Stories of a 20th Century Life ISBN 0-915760-04-5 by WR Sears former assistant to Von Karman Falsehood whether intentional or not is still being taught The most popular theory of wing operation which we may call Hump Theory because it requires a wing to have a more convex upper surface as compared to the lower is easily shown to be false Hump theory is based on an assumption of equal transit times that air passage over a curved upper wing surface must occur in the same length of time as air passage below where the surface is more flat and hence of a shorter path length In order to have the same transit time flow at the longer path upper surface must be of greater velocity than that at the lower surface Thus in accordance with Bernoulli’s law it is reasoned that upper surface pressure must then be less than at the lower surface thereby producing upward lift Equal transit time is sometimes illustrated by representing bits of passing flow above and below an airfoil or wing as shown here: Although Bernoulli’s law is sound and well proven this popular explanation world-wide of wing operation is false Upper surface flow is indeed faster than the lower so much so that upper surface transit time is normally less than the lower as indicated here: Although the assumption of equal transit time is wrong and has no basis in known physics it can be found in books from otherwise reputable publishers such as National Geographic Macmillan and others in this country and abroad College level teaching of aerodynamicists and aeronautical engineers does not include equal transit time which cannot survive mathematical investigation The fallacy of equal transit time can be deduced from consideration of a flat plate which will indeed produce lift as anyone who has handled a sheet of plywood in the wind can testify As indicated in the above figure air approaching the plate such as a sheet of plywood accelerates into the reduced above-plate pressure with increasing velocity while air approaching below is slowed in the increased pressure in accordance with Bernoulli Thus faster upper surface flow can be described as a result of pressure difference rather than the cause of it Bernoulli’s principle law or effect states that velocity varies in inverse relation to pressure but does not assign cause-and-effect relation Unfortunately a great amount of confusion has been generated by abuse of Bernoulli’s law in erroneous cause-and-effect explanations PHYSICAL REALITY Basic Newtonian principles of aerodynamic lift and propulsion include: 1 Upward lift is derived by accelerating air mass downward 2 Forward propulsion of propellers and jets is gained by accelerating air mass rearward 3 Drag is incurred through accelerating air mass forward as by viscous coupling For the most part this is undesirable but unavoidable 4 Air mass recirculates upward at a rate equal to the rate at which it is displaced downward in the lift process thus normal atmospheric mass distribution is maintained Sir Isaac Newton 1642-1727 provided for us laws of physics which govern aerodynamic lift His first law is that velocity of an object or bit of mass changes only when the mass is acted upon by applied force The second law states that when force is applied to a mass it accelerates meaning changes velocity at a rate equal to the force-to-mass ratio The term velocity includes both speed and direction Thus force is required to change direction or speed of an object We drive our cars by operating a system of accelerators These include throttle brakes and steering wheel as indicated here Newton’s third law states that mass resists acceleration with equal and opposite force Thus when a cannon accelerates a missile forward the cannon recoils in rearward direction in response to the opposite force In air or other fluid force is exerted in pressure difference Pressure difference between two positions in air is distributed across the distance between positions as a pressure gradient equal in magnitude at any point to the ratio of pressure difference increment dp to distance increment ds at that point Direction of the pressure gradient is toward the position of greater pressure The direction of air acceleration is toward lower pressure and thus opposite to the direction of pressure gradient A tornado has reduced air pressure at its center Thus air moving around it is accelerated toward reduced center pressure centripetal acceleration so that it continuously curves around the center In opposition with equal counter-centripetal force air pushes outward from the center producing an outward pressure gradient which we may refer to as a centrifugal pressure gradient In a demonstration sometimes wrongly described as showing lift due to pressure reduction in moving air or pressure reduction due to flow path restriction a ball or balloon is suspended by a jet of air Such demonstration may be designed for the purpose of attracting attention to the vacuum cleaner section of a department store In reality flow follows the surface of the ball in a phenomenon known as surface attachment or Coanda effect producing in the curving flow an outward pressure gradient which reduces surface pressure so that the ball is drawn into the flow Coanda effect can be demonstrated by the device shown in the following photograph In demonstration the coffee can is removed and the candle lighted The small blower is then energized blowing air above the candle with no significant effect on the flame The blower is then stopped and the coffee can is inserted as shown When the blower is again energized flow follows the can surface downward and extinguishes the candle flame The blower shown was found in an electronics flea market Perhaps a small hair dryer operating at low speed would suffice for one wishing to produce a similar demonstration A popular classroom demonstration of aerodynamic lift involves a drooping sheet of paper as in figure A which lifts when air is blown over it This is often mistakenly described as pressure reduction in moving air according to to Bernoulli principle In reality it is another demonstration of pressure reduction in a centrifugal pressure gradient The upward pressure gradient in downwardly accelerating flow opposes atmospheric pressure resulting in upper surface pressure reduction and lift If the lift in figure A were caused by Bernoulli principle then the paper in figure B should droop further when air is blown beneath it However as shown it raises when the upward pressure gradient in downward-curving flow adds to atmospheric pressure at the paper lower surface Curvature of flow and resulting pressure gradients is the source of lift for heavier than air aerodynamic flight Description of lift as due to pressure gradients in curvature of above-wing and below-wing flows was given by Otto Lilienthal in his 1889 book titled Birdflight as the Basis of Aviation Unfortunately that has been neglected for well over a century in favor of pseudoscientific descriptions Lilienthal found that wing curvature from leading edge to trailing edge improved efficiency by adapting the wing to required curvature of flow CIRCULATION Higher level theory involves Bernoulli in lift explanation but attributes difference in velocity between above-wing and below-wing flows to circulation rather than equal transit time All air movements are circulatory Circulation begins to develop when an airfoil is started into forward motion As indicated in the next figure air following the surfaces is accelerated downward Because of downward acceleration pressure below the airfoil is increased and pressure above it is decreased In beginning response to the pressure difference upward recirculation of air mass occurs around the leading and trailing edges As forward airfoil movement continues downward movement left behind the trailing edge divides into forward and rearward rotational patterns carrying upward recirculation around the aifoil and around the center of a receding aft vortex created in shear between downward and upward movements Forward recirculation around the airfoil is known as circulation while the recirculation pattern left behind is known as the starting vortex The principle of conservation of angular momentum demands angular momenta of the two rotational patterns be equal and opposite In equivalent classical teaching Helmholtz’ vortex theorems require equal and opposite vorticity Circulation is regenerative Pressure difference produces circulation and as circulation upward momentum ahead is intercepted by wing forward movement and recurved downward more pressure difference is produced Thus circulation increases regeneratively until reaching the limit at which it provides downward movement at the rear for flow to depart the trailing edge in the pointed direction Circulation in excess of this would be opposed by airfoil direction With mature circulation providing downward movement matching the need for flow departure in the pointed direction at the trailing edge participation of the starting vortex is no longer needed or accepted and growths of starting vortex and forward circulation cease In this mature and stable circulation state called the Kutta condition theoretical lossless lift of an airfoil is equal to the rate of circulation downward momentum produced plus an equal rate of upward recirculation momentum intercepted by airfoil forward movement For derivation of the equation for ideal lift without losses in two-dimensional flow click here Evidence of the regenerative nature of circulation and lift can be sometimes be seen in flow downstream from bridge pilings A round piling has no departing flow direction limiting feature This allows circulation and lift to increase until stall occurs which is followed by lift reversal due to effects of residual boundary layer and pressures A trail of alternating vortices occurs as circulation and lift periodically produce stall and reversal In each reversal of circulation and lift a new starting vortex is carried downstream For more on regeneration of circulation of round cylinders click here Circulation is sometimes described as a rotational movement added to or superimposed on passing flow With the added rotational component oncoming flow can be described as rising upward ahead of an airfoil or wing descending behind having increased velocity above and decreased velociy below Alternatively circulation can be considered as a transitory rotation of air which travels with the airfoil or wing through otherwise relatively still air in a manner somewhat analogous to movement of water as it is parted laterally by a ship bow and rejoins at the stern in semicircular-like movements Such water movements travel with the ship even though the water does not The concept of superimposed circulation may seem a bit abstract but circulation is a real rotational movement that travels through the air with the wings of an airplane in flight Alternate perspectives of fixed wing in passing flow and moving wing through still air are illustrated below In passing flow perspective flow curves upward ahead of the lifting wing or airfoil toward reduced pressure above and away from increased pressure below accelerates rearward into reduced above-wing pressure with increased rearward velocity and decreases rearward velocity below in response to increased pressure At the trailing edge upper and lower flows merge to leave in downward direction pointed by the airfoil Merging into common direction and pressure upper and lower flow velocities difference ceases to exist Downward departing flow curves upward toward being parallel to the path of flight again in response to gradients of decreased above-wing pressure and increased below-wing pressure In stable flight circulation kinetic energy is recovered from flow left behind and imparted to oncoming flow ahead through fore and aft pressure gradient couples and Bernoullian process In lossless stable two-dimensional lift condition equal opposing forward and rearward couples have no net effect on circulation angular momentum which remains constant If angle of attack is increased then downwash demand is increased for re-establishing the Kutta condition Increase is momentarily shared between circulation increase and production of a new starting vortex Angular momentum increase in wing circulation is matched by equal and opposite angular momentum production in the new vortex When the Kutta condition is again established growths of starting vortex and circulation cease and the starting vortex no longer involved with circulation is left downstream In the opposite case of decreased angle of attack a portion of airfoil circulation in excess of that permitted by the Kutta condition is deflected by the airfoil and carried downstream as a new vortex of angular momentum equal in magnitude to that of circulation reduction and of same direction as circulation In two-dimensional flow of wind-tunnel wall-to-wall confinement of an airfoil section circulation and starting vortex develop as two opposite-direction vortices formed in division of downwardly driven air into forward and rearward paths of upward recirculation In this case the stabilized rate of upward recirculation ahead is equal to the rate of downward displacement left behind In the case of a real wing beginning lift in open air where there is no barrier to lateral loss around wing ends circulation starting vortex and wing end vortices are all part of a closed loop of recirculation vorticity as indicated below formed around downwardly accelerated air behind a wing Conservation of angular momentum is maintained as angular momentum on one side of the vortex loop is matched by equal and opposite direction angular momentum on the other side For our purposes air driven downward by a wing or airfoil will be termed downwash and upward recirculation will be termed upwash For a real wing in stable flight upwash is driven upward ahead of the wing and around wing ends by pressure difference between above-wing and below-wing regions Upwash around wing ends in combination with downwash left behind produces twin vortices which trail behind wing ends The starting vortex portion of the initial loop is left behind at the beginning of trailing vortices and has virtually no lasting involvement in wing performanceContinuous production of trailing vortices involves continuous energy loss A helicopter which also derives lift from producing downwash leaves a similar pair of trailing vortices behind but helicopters can fly much slower than fixed-wing craft of comparable weight and thus can leave much more intense vortices The following picture courtesy of NASA shows a vertical column of red smoke drawn into a recirculating downwash vortex left behind a slow flying fixed-wing cropduster If circulation changes as necessary to accommodate changing lift conditions strength of trailing vortices equally changes and a new starting vortex is left behind equal and opposite in angular momentum to change of angular momentum in wing circulation Thus conservation of angular momentum is maintained The body of downward air propulsion and upward recirculation is extensive involving a large rate of air mass in wing circulation and trailing vortices Inertial opposition of the large rate of air mass involvement to downward acceleration allows the required rate of downward air mass acceleration required for lift to be achieved with a sufficiently low effective downward velocity that the rate of kinetic energy input proportional to mass times velocity squared production is small enough for practical flight In level flight the downward momentum produced in lift is ultimately intercepted by the Earth’s surface thereby transferring airplane weight to the surface However a steeply banked turn may produce mostly horizontal momentum which would be retained in continued atmospheric movement That wing lift can be effective in maneuvers of any attitude demonstrates the fact that lift at altitude is in reaction to acceleration in a large mass of circulation not necessarily dependent on weight force transfer to the ground Pressure gradients which produce rising circulation ahead of a wing also produce upward recirculation around wing ends leaving trailing vortices behind In wind tunnel two-dimensional flow airfoil tests trailing vortices are totally prevented by confinement of tunnel walls For a wing operating in open air limited confinement is effected by inertial resistance of surrounding air mass to acceleration Longer wingspan produces greater lateral path length with greater mass inertial resistance to lateral acceleration and associated performance loss The following figure indicates relative lift performances of equal area wings with differing span Differing span is represented in differing aspect ratio the ratio of span to chord INDUCED DRAG The lift process also produces forward and rearward thrust components as the force vectors here indicate Forward thrust results from leading edge pressure reduction in the rearward curvature of rising circulation ahead This pressure reduction known as leading edge suction varies with the ratio of lift to airspeed and on some airplanes is piped from a leading edge port as shown below to a stall warning horn in the cabin Another type of stall sensor actuates when intense circulation around the wing leading edge lifts a small vane also shown which operates a switch connected to an electrical warning horn In reaction to downward and forward acceleration in redirection of circulation at the rear of a wing lift and rearward thrust are generated If there were no losses the rearward thrust would be equal to the forward thrust generated in leading edge suction associated with circulation However the rate of rising air mass ahead of a wing is less than that of descending air mass behind because of partial upward circulation diversion into lateral loss paths around wing ends Thus in addition to loss of lift lateral loss around wing ends causes drag known as induced drag due to rearward thrust of aft circulation redirection being greater than the forward thrust Longer wing spans which provide greater air mass inertial opposition to lateral loss are aerodynamically desirable but limiting factors are stress due to bending moments and space requirements for ground operations and hangaring For special airplanes like Voyager which flew around the world non-stop and unrefueled and the U2 spyplane which flew long high altitude missions long spans were vital Circulation theory is traditionally presented in a highly mathematical context for aerodynamicists and aeronautical engineers as appropriate for aircraft engineering Mathematical complexity and departure from Newtonian logic reduces the utility of most circulation theory books for individuals seeking basic conceptual understanding of wings and aircraft configuration When it was found that mathematical expression of vortex flow velocity involved the same equation as that expressing magnetic field strength induced by electrical current trailing vortices behind wing ends became described as induced The physical basis for induced flow velocity is elusive and authors have written different concepts of it based on differing interpretations of a statement of Ludwig Prandtl 1875-1953 Prandtl stated: The wingtip vortices cause a downward motion of air at the wing which will be shown to be responsible for the drag From Prandtl-Tietjens APPLIED HYDRO-AND AEROMECHANICS ISBN 0-486-60375-X Clark Millikan in Aerodynamics of the Airplane Wiley and Sons 1941 states: The numerical computations in Prandtl’s theory are identical with calculations developed long ago in the theory of electromagnetic induction so that the adjective induced has been taken from the electrical field and introduced into aerodynamics John D Anderson Jr in Fundamentals of Aerodynamics 2nd edition 1991 ISBN 0-07-001679-8 says of trailing vortices: The two vortices tend to drag the surrounding air with them and this secondary movement induces a small velocity component in the downward direction at the wing Krisnamurty Karamcheti in Principles of Ideal Fluid Aerodynamics 1996 ISBN 0-089874-113-0 writes: It is customary to refer to the velocity at any point in the vortex flow as the velocity induced by the vortex It must be understood that this is simply a matter of convenience and does not mean that the vortex is actually causing the flow for they just coexist Clearly there is no common understanding of what induction or induced really means in aerodynamics context Reasonably that is because the concept is at best an analogy TILT OF LIFT DIRECTION In classical aerodynamics which regards trailing vortices as induced from central cores rather than caused by lateral loss of circulation upward movement addition of induced downwash to flow is said to cause downward tilt of flow passing the wing With lift being aerodynamic force perpendicular to relative flow downward tilt of passing flow can account for a rearward tilt of lift illustrated symbolically below The rearward component of tilted lift appears as drag known as induced drag because it is said to be caused by induction effects The concept of drag due to downward tilt of passing flow has merit but the tilt can better be described not as due to addition of induced downwash but due to loss of circulation upwash ahead of the wing as it partially diverts into lateral paths around wing ends making the upwash arrival angle at the wing leading edge less steep than the downwash angle at the trailing edge A MODEST PROPOSAL Obfuscation might have been avoided if the electromagnetism analogy of induction not been so convenient as a substitute for Newtonian reasoning Trailing vortices as well as other aspects of aerodynamic lift should be accounted for in Newtonian terms as proposed by Dr Jaako Hoffren of Helsinki University of Technology in paper AIAA 2001-0872 Quest for an Improved Explanation of Lift presented at the Aerospace Sciences Meeting amp Exhibit in Reno in 2001 If aerodynamics were commonly described in Newtonian terms omitting induction and regarding all air mass accelerations as associated with forces of pressure difference then high school and early college physics and mathematics courses might be enriched with basic aerodynamics topics and false teaching of equal transit time half-venturi theory and electromagnetism-like induction might be abandoned Another questionable point in classical teaching is that of describing circulation as a reaction to the starting vortex which is usually said to be caused by viscous drag at the wing trailing edge The reasoning involves theorems of vorticity which require that creation of any vortex be accompanied by other opposite and equal vorticity This is equivalent to the Newtonian principle of conservation of angular momentum This requirement does not justify assumption that one vortex causes another of opposite rotation as indicated by some authors Starting vortex and circulation can readily be explained as created simultaneously in equal and opposite reactions to pressure gradients occurring when air mass is displaced by airfoil movement Two books by this author present basic aerodynamics in terms of Newtonian principles as taught in high schools and college with circulation but with less mathematical complexity than usually associated with circulation teaching The first soft cover Stop Abusing Bernoulli!- How Airplanes Really Fly ISBN 0-9646806-2-9 with 160 pages and 100 illustrations as shown on the left was published in 1999 The book has sold to many countries around the world and as shown on the right has been translated into Korean by Dr SK Lee for use in Pusan National University Peter Garrison’s review of Stop Abusing Bernoulli! in July 1999 FLYING MAGAZINE recommends it to all pilots and would-be pilots However it is no longer in print although a few copies are still on the shelf at Academy of Model aeronautics AMA museum phone 765-289-4236 A second book with more detail better illustrations and hardcover incorporates the Stop Abusing Bernoulli information and more The second book Introduction to Aerodynamics ISBN 0-9646806-3-7 published in January 2003 with 224 pages and 167 illustrations begins with applicable principles of Newtonian physics explained in aerodynamics context followed by aerodynamics of wings airplanes helicopters and surface effect craft explained in Newtonian context For information and reviews posted on Amazoncom click here Introduction to Aerodynamics list 2950 is available from Amazoncom from Academy of Model Aeronautics AMAMuseum bookstore at Muncie IndianaPhone 765-289-4236 and from DAR Corporation in Lawrence KS phone 785-832-0434 e-mail email protected For further information contact the publisher: Regenerative Press 1900 Romine Road Anderson IN 46011 Discount available for libraries and educational institutions Author Gale Craig is retired from 35 years in General Motors Research and development He holds the MS degree in physics is licensed to teach high school physics and mathematics is named as sole inventor in nineteen US patents in widely varied areas and is a licensed pilot of 1700 hours who owns and flies a Cessna 182 NOTE: The author again presented a one hour forum on this topic during the 2009 Experimental Aircraft Association convention at Oshkosh WI Thursday July 30 at 1:00 PM in the 001 Dake Corporation Pavilion For a copy of the handout go to regenpresscom/handoutpdf Comments or questions cCopyright 2003 2008 Gale M Craig This web page may be reproduced in unbound form for non-profit individual or classroom use if printed in entirety with copyright notice |
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NASA объявит первые экипажи космических аппаратов SpaceX Crew Dragon и Boeing CST-100 Starliner |
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